Return Air with Portable Heat Pump

I have a Haire 9500 BTU portable heat pump that I use in my insulated garage. The back of the unit has two 5" hoses. During A/C usage, as the manual states, they are both vented out of the garage. The manual does not give instruction on how they should be used during heating. I have been running the exhaust hose out as normal with the intake pulling return air from inside the garage. The unit keeps the garage at 62-65F when outside air temps are 10's and 20's.

Is there greater heat transfer capability with the air intake coming from outside (both hoses vented outside) or am I better off the way I'm running now?

EDIT: After reading the manual of a similar unit, it requires both hoses be vented outside. Why Haier doesn't mention this in their manual is beyond me.